Make it a challenge - no serious terraforming. Conform to the flow of the hillsides - make those your template.
I actually have OCD (or maybe OCPD; it feels like a combination of both), but I rarely make any changes to the terrain once I've got my entrance dug out. If it weren't for the stone double doors and the rampant deforestation, you'd never know a fortress was there. However, I still make sure all my rooms are the exact same size and get angry when I have to choose between mining a chunk of valuable ore and not having an odd hole in the wall.
My advice is that when an unexpected terrain feature derails your carefully laid plans, spend a few minutes looking at said terrain feature. I often find that what at first looks like an architectural disaster actually turns out to be the cornerstone of one of the more awesome parts of my fortress.
For example, my first ever fortress was dug out of shale that contained large amounts of jet. The colors are very different, so I was very careful not to make any room with mismatched walls. Imagine my horror when, in the center of what was to be the main living quarters, I struck
cinnabar. It was a disaster! But I really needed those rooms, and they were already half excavated, so I kept digging. I ended up with a large block of bedrooms with a thin vein of cinnabar cutting through it diagonally like a gash. The effect was quite striking, and it became my favorite spot in the fortress. All my best dwarves got half-cinnabar rooms.
On a later fortress, I experimented with building a large, subterranean tower. It was circular, with stairwells in each corner and a hollow area in the middle that was to contain a waterfall. Large hallways would branch off on each side, leading to workshops or living quarters, or whatever else that floor held. Halfway down, my miners excavating the eastern wall strike hematite. I couldn't just leave it there; it's
iron. So I mined out the entire vein, and it became very clear that the symmetry of the tower was absolutely ruined. I looked at for a while, but I only slowly realized what I had: It was a long, irregular, jaggedy hallway with a bright red floor in one of the deepest areas of my fortress. I knew exactly what needed to be done.
The tunnel left behind from mining the hematite vein would become the entrance hallway to the fortress crypt. All great dwarves, whether they be struck down by battle, famine, or lifelong toil, would make their final journey down the Path of Iron Blood and be laid to rest in the Great Burial Halls beyond.
Just the thought of it is way cooler than any big symmetrical tower would be.